1. Field of the Invention
The present invention relates to ink jet printers. Particularly, the invention relates to the control of ink droplets to ensure proper registration on a recording medium.
2. Prior Art
The use of ink jet printers for printing data and other information on a strip of recording media is well known in the prior art. Conventional ink jet printers incorporate a plurality of electrical components and fluidic components. The components coact to enable the printing function. The fluidic components include a print head having a chamber for storing a printing fluid or ink and a nozzle plate with one or more ink nozzles interconnected to the chamber. A gutter assembly is positioned downstream from the nozzle plate in the flight path of ink droplets. The gutter assembly catches ink droplets which are not needed for printing on the recording medium.
In order to create the ink droplets, a drop generator is associated with the print head. The drop generator vibrates the head at a frequency which forces the thread-like streams of ink, which are initially ejected from the nozzles, to be broken up into a series of ink droplets at a point within the vicinity of the nozzle plate. A charge electrode is positioned along the flight path of the ink droplets. The function of the charge electrode is to selectively charge the ink droplets as said droplets pass said electrodes. A pair of deflection plates is positioned downstream from the charge electrodes. The function of the deflection plates is to deflect a charged ink droplet either into the gutter or onto the recording media.
One of the problems associated with ink jet printers of the aforementioned type is that of ink droplets misregistration at the recording surface. The ink droplets misregistration arises from interaction between the droplets as said droplets are propelled along a flight path towards the recording surface. The causes for droplets interaction are usually twofold, namely: the aerodynamic drag on the respective droplets and the electrical interaction between the electrical charges which are placed on the ink droplets.
The aerodynamic interaction and the electrical interaction are closely related. In fact, the aerodynamic interaction and the electrical interaction are complementary and are usually never observed independently. As ink droplets are generated at the nozzle plate, the charge electrode deposits a certain quantum of electrical charge on the droplets. Depending on the polarity of the charge, the droplets either repel or attract one another. The electrical forces which attract and/or repel the ink droplets tend to affect the relative spacing between the droplets. As such, some droplets arrive at the recording media early while others arrive late. In some situations, the droplets arrive at the recording media in groups rather than individual drops. The net result is that the copy quality is relatively poor due to droplet misplacement on the media.
The aerodynamic interaction also tends to affect the relative spacing between droplets. Spacing is affected because the aerodynamic interaction either increases or decreases the velocity of the droplets. As a result, some ink droplets are reaching the media early while others are reaching the media late. The overall effect is that the presence of the aerodynamic interaction also called the aerodynamic drag, aggravate or magnify the effect of the charge interaction.
In order to effectively solve droplets registration problems, both the charge interaction and the aerodynamic interaction have to be addressed. The prior art uses the so-called guard drop method to solve the charge interaction problem. In this method nonadjacent droplets are charged. Stated another way, charged droplets are separated by a predetermined number of noncharged droplets.
In addressing the aerodynamic interaction problem, the prior art utilizes a gas stream, such as air, to compensate for the aerodynamic drag on the ink droplets. U.S. Pat. No. 3,596,275 is an example of the prior art method. In that patent a stream of air is introduced into the droplet flight path. The air flows collinearly, with the stream of ink droplets and reduce the aerodynamic effect. In order to maintain laminar air flow beginning at the point where the droplets are interjected into the air stream or vice versa, the nozzle is mounted in the center of the air stream. The charging electrode is fabricated in the shape of a hollow streamline strut. The strut is fitted with an opening through which ink droplets are ejected. The strut surrounds the nozzle with its opening and stream line contour position in the direction of air flow. Although this approach appears to be a step in the right direction, one of the main problems is that the air flow is not fully laminar (that is, free from turbulence). Turbulent air flow tends to blow the minute droplets from their normal trajectory and, therefore, the misregistration phenomenon is not completely solved. In fact, turbulent air flow may well aggravate the misrepresentation problem.
Another problem with the above-described patent is that its teaching and apparatus is only effective with a single nozzle head. When a head having a relatively large number of nozzles (that is, a multinozzle head) is used, it would be impractical to build a strut to surround such a head.
U.S. Pat. No. 4,097,872 is another prior art example of an aspirator where a fluid such as air is used to correct for aerodynamic interaction or aerodynamic drag. The aspirator includes a housing having a tunnel therein. The tunnel is spaced from an ink jet nozzle which emits an ink stream which passes through the tunnel. The tunnel is characterized by a circular geometry with a settling chamber section and a flow section. Air turbulence is removed at the settling chamber. Although the teaching in the subject patent works well for its intended purpose and is a significant improvement over the prior art, it suffers from one drawback.
The primary drawback is with the circular geometry, the velocity profile across the tunnel is not constant. Of course, the velocity at the center of the tunnel is constant. Therefore, with a single nozzle head positioned to eject ink in the center of the tunnel, the droplets will experience constant velocity. However, with a multinozzle head, the velocity across the streams will not be constant. Therefore, streams ejected into the tunnel would experience variable velocity. Stated another way, due to the nonuniform velocity profile across the channel, the disclosed device is not suitable for use with a multinozzle head.